Thanks for making this!
Our team got the app and it's pretty cool. My team is probably going to use it this season, to replace the whiteboard we were using for a similar purpose last season.
I have only complaint: You can undo your last few changes or you can clear the field, but otherwise, there isn't really a way to selectively erase. This would be a very useful thing to have. I also agree with Jaxom's idea that this should have an "only one for each category" restriction with regard to the defenses. Otherwise, the app is great, and I love it's simplicity. Very easy to use, and to learn to use.